1. Field of the Invention
The present invention relates to an image reading apparatus and method for reading images of original documents of different sizes.
2. Description of the Related Art
Image reading apparatus have come to be used as the input portion for digital-type copying apparatus, facsimiles and the like. In general, such image reading apparatus have image sensors comprising photoelectric converting elements, and produce electrical signals in accordance with an image on an original document.
In recent years, there has been a demand for image reading apparatus capable of automatically reading both sides of an original document. Image reading apparatus capable of automatically reading a duplex original can be broadly divided into two types.
A first type of image reading apparatus is provided with a pair of sensors, wherein one sensor reads the front side of the original and the other sensor reads the back side of the original. The aforesaid sensors are provided in opposed pairs and read the images on both sides of an original by a single pass of the,duplex original between said sensor pair.
In conventional image reading apparatus, electrical signals output from the sensors are converted to image signals through various processes, e.g., binary processing, halftone processing and the like. The circuits necessary to accomplish the aforesaid processes are generally called signal processing circuits, and have complex circuit constructions. The image signals output from the signal processing circuits are printed by means of a printer, or transmitted by means of a facsimile apparatus.
Japanese Patent Application No. 3-3564 discloses a facsimile apparatus provided with an image reading apparatus of the first type described above. This image reading apparatus has a signal processing circuit respectively provided for each of the two image sensors. As previously mentioned, these signal processing circuits are expensive and, therefore, increase the cost of the facsimile apparatus.
Furthermore, the aforesaid image reading apparatus simultaneously generated image signals for the front side of the original and image signals for the back side of the original. However, since the facsimile apparatus cannot transmit both sets of image signals simultaneously, the apparatus is provided with a memory for temporarily storing the image signals for the back side of the original. That is, the image signals for the front side of the original are simultaneously generated and transmitted, whereas the image signals for the back side of the original are simultaneously generated and stored in memory, and after the image signals for the front side have been completely transmitted, the image signals for the back side which are stored in memory are transmitted. The aforesaid memory is also expensive and, therefore, increases the cost of the facsimile apparatus.
Japanese Patent Application No. 3-265355 discloses an image reading apparatus of the aforesaid first type, and is provided with a pair of image sensors which read the images on both sides of a duplex original by passing said duplex original twice between said pair of sensors. More specifically, one sensor generates electrical signals during the first pass of the original, and the other sensor generates electrical signals during the second pass of the original. Accordingly, the aforesaid apparatus does not simultaneously generate the electrical signals of the front side and the electrical signals of back side of the original, such that the processing of said signals can be accomplished by a single signal processing means. However, the aforesaid apparatus is provided with image reading means specific to the front side and specific to the back side of the original which requires two passes of the original document, thereby slowing the image reading speed.
A second type of image reading apparatus requires that an original document be fed twice to be read by a single sensor. The image of the front side of the original is read on the first pass of the original, whereupon the original is inverted and the original is fed again, and thereafter the image of the back side of the original is read on the second pass of the original.
Japanese Patent Application No. 2-63262 discloses an image reading apparatus of the aforesaid second type. This image reading apparatus is provided with a stationary sensor, an original feeding portion on one side of said sensor, and an original inverting portion on the other side of said sensor. The sensor reads the front surface of the original as said original is transported in a first direction (from the original feed portion toward the inverting portion). After this reading the original is discharged to the original inverting portion and inverted front-to-back, then the inverted original is transported in a second direction (from the original inverting portion toward the original feeding portion). The sensor reads the second surface of the original being transported in said second direction.
The aforesaid type of image reading apparatus can be produced less expensively because only one sensor is used. However, the aforesaid image reading apparatus has an original inverting path of fixed length causing the positional relationship between the sensor and the inverting path to be fixed, such that in the case of an original of a large size and in the case of an original of a small size, the original inverting path to be transited must remain constant, thereby adversely affecting efficiency.